1. Field of the Invention
The invention relates to complementary transistor amplifier output stages, and relates more particularly to circuits for maintaining constant bias current through the output transistors of the complementary transistor output stage to reduce or eliminate crossover distortion.
2. Brief Description of the Prior Art
Complementary transistor amplifier output stages including an NPN output transistor and a PNP output transistor coupled in series between two power supply voltages are well known. Biasing schemes to produce a bias voltage between the bases of the NPN output transistor and the PNP output transistor in order to maintain a bias current in the PNP and NPN output transistors are also known. Such bias currents are highly desirable in order to provide high fidelity output signals with a minimum amount of crossover distortion. Crossover distortion occurs when the voltage produced by the bias circuit does not properly match the emitter-to-base voltage drops of the output transistors. One common bias circuit consists essentially of compensating diodes connected in series between the base electrodes of the NPN and PNP output transistors. The bias voltage provided by that bias circuit is not readily adjustable. The forward voltage drop of the diodes must be carefully matched to the emitter-base voltages of the output transistors (hereinafter called "V.sub.BE voltages") in order to effect proper operation. Although the above diode bias circuit produces a quiescent bias circuit through the complementary output transistors, a serious "thermal runaway" problem may occur if one of the NPN or PNP output transistors heats up as a result of, for example, delivering a large signal current to a load connected to the output. The thermal runaway problem occurs because the emitter-base PN junction of that one of the output transistors heats up rapidly. Since the V.sub.BE voltage of a bipolar transistor decreases with temperature, the heating up of the emitter-base junction with a constant V.sub.BE bias voltage applied thereto causes yet more quiescent bias current to flow through the transistor, which in turn causes still further heating of the emitter-base junction. If there is no mechanism or element to limit the current, catastrophic failure of the emitter-base junction occurs. Thermal feedback between the output transistor and the diodes in the diode bias circuit may have a long time constant and poor .DELTA.V.sub.BE /.DELTA.T matching to the output transistors, where .DELTA.T is the change in junction temperature of the diodes. Consequently, known diode bias circuits are relatively inadequate in compensating for the temperature-caused decrease in the V.sub.BE voltage of the output transistors as the temperature of the output transistors increases. Another known biasing circuit called a V.sub.BE multiplier has been connected between the bases of the PNP and NPN complementary output transistors to maintain a bias current therein. A typical V.sub.BE multiplier circuit includes an NPN transistor having a resistor connected between its base and collector and another resistor connected between its base and emitter. Although the V.sub.BE multiplier may be adjusted (by adjusting the resistor values) to provide a desired bias current in the PNP and NPN complementary output transistors, it nevertheless has generally the same poor thermal tracking and .DELTA.V.sub.BE /.DELTA.T matching problems as the above-described diode bias circuit. If constant bias current is to be maintained in the NPN and PNP output transistors, the bias voltage must also decrease with junction temperature of the output transistor at the same rate as the V.sub.BE voltages of the output transistor. The thermal feedback schemes of the prior art, which involve approaches such as using a common heat sink for the biasing circuit and the output transistors, do not satisfactorily maintain a constant bias current. Regenerative increasing of the bias current and output transistor junction temperature may result in catastrophic failure of the output stage.